Description:
A vulnerability was reported in Microsoft Skype. A local user can obtain elevated privileges on the target system.

A local user can place a specially crafted DLL in the '[SystemRoot]\Temp\' directory. When the Skype update mechanism ('[ProgramFiles]\Skype\Updater\Updater.exe') runs, arbitrary code will be executed with System privileges.

The vendor was notified on September 2, 2017.

Stefan Kanthak reported this vulnerability.

Impact:
A local user can obtain elevated privileges on the target system.

Subject: Defense in depth -- the Microsoft way (part 51): Skype's home-grown updater allows escalation of privilege to SYSTEM

Hi @ll,
since about two or three years now, Microsoft offers Skype as
optional update on Windows/Microsoft Update.
JFTR: for Microsoft's euphemistic use of "update" see
<http://seclists.org/fulldisclosure/2018/Feb/17>
Once installed, Skype uses its own proprietary update mechanism
instead of Windows/Microsoft Update: Skype periodically runs
"%ProgramFiles%\Skype\Updater\Updater.exe"
under the SYSTEM account.
When an update is available, Updater.exe copies/extracts another
executable as "%SystemRoot%\Temp\SKY<abcd>.tmp" and executes it
using the command line
"%SystemRoot%\Temp\SKY<abcd>.tmp" /QUIET
This executable is vulnerable to DLL hijacking: it loads at least
UXTheme.dll from its application directory %SystemRoot%\Temp\
instead from Windows' system directory.
An unprivileged (local) user who is able to place UXTheme.dll or
any of the other DLLs loaded by the vulnerable executable in
%SystemRoot%\Temp\ gains escalation of privilege to the SYSTEM
account.
The attack vector is well-known and well-documented as CAPEC-471:
<https://capec.mitre.org/data/definitions/471.html>
Microsoft published plenty advice/guidance to avoid this beginner's
error: <https://msdn.microsoft.com/en-us/library/ff919712.aspx>,
<https://technet.microsoft.com/en-us/library/2269637.aspx>,
<https://support.microsoft.com/en-us/help/2389418/secure-loading-of-libraries-to-prevent-dll-preloading-attacks>
and
<https://blogs.technet.microsoft.com/srd/2014/05/13/load-library-safely/>
... which their own developers and their QA but seem to ignore!
See <https://bugs.chromium.org/p/project-zero/issues/detail?id=440>
for the same vulnerability in another Microsoft product!
stay tuned
Stefan Kanthak
Timeline:
~~~~~~~~~
2017-09-02 vulnerability report sent to vendor
2017-09-03 reply from vendor: "MSRC case 40550 opened"
2017-09-06 notification from vendor's case manager: "report passed
to product group for investigation"
2017-10-27 reply from vendor's case manager:
"The engineers provided me with an update on this case.
They've reviewed the code and were able to reproduce
the issue, but have determined that the fix will be
implemented in a newer version of the product rather
than a security update. The team is planning on shipping
a newer version of the client, and this current version
will slowly be deprecated. The installer would need a
large code revision to prevent DLL injection, but all
resources have been put toward development of the new
client."
2018-02-09 report published